1. Field of the Invention
This invention relates to an output torque control system for internal combustion engines for vehicles, which is capable of reducing a shock generated during gear shifting of an automatic transmission of the engine.
2. Prior Art
Conventionally, an output torque control system for an internal combustion engine for vehicles is known, which reduces output torque from the engine during gear shifting of an automatic transmission of the engine such as an upshift in an inertia phase extending from a time point at which a clutch of the transmission selected before the upshift (lower-speed clutch) starts to be disengaged to a time point at which the upshift is completed, by changing the opening of a throttle valve of the engine, to thereby reduce a shock generated during gear shifting of the automotive transmission. In the known output torque control system, control of reducing the engine output torque is started at the time of start of disengagement of the clutch, and the timing of termination of the engine output torque-reducing control is determined according to a ratio of the rotational speed of an output side (counter shaft) of a gear mechanism of the automatic transmission to the rotational speed of an input side (main shaft) of the same (hereinafter referred to as "input/output rotational speed ratio"), in a manner dependent upon a delay in recovery of the actual engine output torque from a value reduced by the engine output torque-reducing control.
More specifically, according to the known output torque control system, the timing of termination of the engine output torque-reducing control is determined such that a time point which the actual engine output torque recovers a value assumed immediately before the gear shifting coincides with a time point at which the input/output rotational speed if ratio recovers a value assumed immediately before the gear shifting (engage point) after termination of the engine output torque-reducing control. For example, en the timing of termination of the engine output torque-reducing control is set to a time point at which the input/output rotational speed ratio becomes equal to a predetermined fixed value (e.g. 0.8) which is set based on normal traveling conditions of the vehicle. According to this control manner, a smooth change in the actual engine output torque can be obtained after termination of the engine output torque-reducing control to reduce a shock generated upon termination of the gear shifting insofar as the vehicle is traveling under normal traveling conditions.
The known output torque control system, which thus determines the timing of termination of the engine output torque-reducing control, based on the input/output rotational speed ratio alone, however, can provide inconveniences depending upon a manner of change in the traveling speed of the vehicle. For example, in the case where gear shifting is effected when the vehicle speed is changing, such as when the vehicle is climbing or descending a slope or is suddenly braked to be sharply decelerated, the time period required to elapse from the time point of start of gear shifting to the time point at which the input/output rotational speed ratio reaches the engage point differs from that obtained when the vehicle is traveling under normal traveling conditions. This will now be explained with reference to FIGS. 1A and 1B showing cases where an upshift is effected when the vehicle is descending a slope. FIGS. 1A and 1B show changes in the driving force of a vehicle caused by the engine output torque-reducing control during an upshift, in which FIG. 1A shows a case where the vehicle speed is slowly increasing, and FIG. 1B a case where the vehicle speed is sharply increasing (the rate of change in the vehicle speed is large). In the case shown in FIG. 1A, a time point t1 at which the input/output rotational speed ratio reaches the engage point and a time point t3 at which the actual engine output torque recovers a value assumed immediately before the upshift almost coincide with each other, and therefore the driving force gently changes, resulting in only a small shock upon termination of the gear shifting. In contrast, in the case shown in FIG. 1B, while the time point t3 at which the actual engine output torque recovers a value assumed immediately before the upshift is almost the same with that in the case of FIG. 1A, a time point t2 at which the input/output rotational speed ratio reaches the engage point is earlier compared with the time point t1 in FIG. 1A. As a result, the driving force sharply drops after the time point t2, resulting in a large shock.
As noted above, when the vehicle is descending a slope, generally the rate of change (rise) in the vehicle speed is larger than a value assumed under normal traveling conditions of the vehicle, and accordingly the rate of change in the rotational speed of the output side of the automatic transmission relative to that of the rotational speed of the input side becomes large so that the engage point is reached at earlier timing. However, the time period elapsing from the time point of termination of the engine output torque-reducing control to the time point at which the actual engine output torque recovers a value immediately before the gear shifting remains almost constant irrespective of a change in the vehicle speed. Therefore, if the timing of termination of the engine output torque-reducing control is set to the time point at which the input/output rotational speed ratio reaches the predetermined fixed value, the time point at which the actual engine output torque recovers becomes later than the engage point, which results in a shock due to a sudden drop in the driving force caused by the reduced engine output torque.
A similar phenomenon to the above described one can occur also when a downshift is effected when the vehicle is climbing a slope, i.e. the time point at which the actual engine output torque recovers becomes later than the engage point, resulting in a shock upon termination of the downshift.
As explained above, the known output torque control system for internal combustion engines for vehicles has the disadvantage that a large shock can be generated upon termination of gear shifting, depending upon a manner of change in the vehicle speed occurring at the gear shifting.